Hearing assistance devices, such as hearing aids, include electronic instruments worn in or around the ear that compensate for hearing losses by amplifying and processing sound. The electronic circuitry of the device is contained within a housing that is commonly either placed in the external ear canal and/or behind the ear. Transducers for converting sound to an electrical signal and vice-versa may be integrated into the housing or external to it.
Whether due to a conduction deficit or sensorineural damage, hearing loss in most patients occurs non-uniformly over the audio frequency range, most commonly at high frequencies. Hearing aids may be designed to compensate for such hearing deficits by amplifying received sound in a frequency-specific manner, thus acting as a kind of acoustic equalizer that compensates for the abnormal frequency response of the impaired ear. Adjusting a hearing aid's frequency specific amplification characteristics to achieve a desired level of compensation for an individual patient is referred to as fitting the hearing aid. One common way of fitting a hearing aid is to measure hearing loss, apply a fitting algorithm, and fine-tune the hearing aid parameters.
Hearing assistance devices also use a dynamic range adjustment, called dynamic range compression, which controls the level of sound sent to the ear of the patient to normalize the loudness of sound in specific frequency regions. The gain that is provided at a given frequency is controlled by the level of sound in that frequency region (the amount of frequency specificity is determined by the filters in the multiband compression design). When properly used, compression adjusts the level of a sound at a given frequency such that its loudness is similar to that for a normal hearing person without a hearing aid. There are other fitting philosophies, but they all prescribe a certain gain for a certain input level at each frequency. It is well known that the application of the prescribed gain for a given input level is affected by time constants of the compressor. What is less well understood is that the prescription can break down when there are two or more simultaneous sounds in the same frequency region. The two sounds may be at two different levels, and therefore each should receive different gain for each to be perceived at their own necessary loudness. Because only one gain value can be prescribed by the hearing aid, however, at most one sound can receive the appropriate gain, providing the second sound with the less than desired sound level and resulting loudness.
This phenomenon is illustrated in the following figures. FIG. 1 shows the levels of two different sounds out of a filter centered at 1 kHz—in this example, the two sounds are two different speech samples. The samples are overlaid on FIG. 1 and one is in a thick dark line 1 and the second is in a thin line 2.
FIG. 2 shows the gains that would be applied to those two different sounds at 1 kHz if they were to be presented to a hypothetical multiband dynamic range compressor. Notice that the ideal gain for each speech sample is different. Again, the samples from the thick dark line 1 are shown in comparison to those of the thin line 2.
FIG. 3 shows the two gains from FIG. 1 represented by the thick dark line 1 and the thin line 2, but with a line of intermediate thickness 3 which shows the gain that is applied when the two sounds are mixed together before being sent to the multiband compressor. Notice that when the two sounds are mixed together, neither receives the exact gain that should be prescribed for each separately; in fact, there are times when the gain should be high for one speech sample, but it is low because the gain is controlled by the level of the mix of the two sounds, not the level of each sound individually. This can cause artificial envelope fluctuations in each sound, described as comodulation or cross modulation by Stone and Moore (Stone, M. A., and Moore, B. C. (2008). “Effects of spectro-temporal modulation changes produced by multi-channel compression on intelligibility in a competing-speech task,” J Acoust Soc Am 123, 1063-1076.)
This could be particularly problematic with music and other acoustic sound mixes such as the soundtrack to a Dolby 5.1 movie, where signals of significantly different levels are mixed together with the goal of provided a specific aural experience. If the mix is sent to a compressor and improper gains are applied to the different sounds, then the auditory experience is negatively affected and is not the experience intended by the produce of the sound. In the case of music, the gain for each musical instrument is not correct, and the gain to one instrument might be quite different than it would be if the instrument were played in isolation. The impact is three-fold: the loudness of that instrument is not normal for the hearing aid listener (it may be too soft, for example), distortion to the temporal envelope of that instrument can occur, and interaural-level difference (ILD) cues for sound source localization and segregation can be distorted, making the perceived auditory image of that instrument fluctuate in a way that was not in the original recording.
Another example is when the accompanying instrumental tracks in a movie soundtrack have substantial energy then compression can overly reduce the overall level and distort the ILD of the simultaneous vocal tracks, diminishing the ability of the wearer to enjoy the mix of instrumental and vocal sound and even to hear and understand the vocal track. Thus, there is a need in the art for improved compression and mixing systems for hearing assistance devices.